The subject of the invention is a gas discharge lamp for DC operation having a double electrode arrangement and a discharge-tight cross connection of the electrode spaces. The electrode spaces are separated by means of a formed part and are hermetically sealed from surrounding areas via a solder glass seal.
It is principally known that the light yield from gas discharge lamps can be increased when the gas discharge lamps are operated with DC instead of AC current. Lately, a number of embodiments of gas discharge lamps for DC operation have been suggested (for example, German Pat. Nos. 2,254,709 and 2,319,401).
Essential characteristics of this known embodiment is that the lamp shape is structured such that the two electrodes are closely adjacent and that the two electrode spaces are gas or metal vapor permeable but are connected in discharge-tight fashion with one another in order to avoid a dropping of the light yield during the operation due to cataphoresis phenomenona. In accordance with the German Pat. No. 2,319,401, these characteristics are obtained by use of an almost closed shape of the tube-shaped discharge space and by means of a capillary connection or a porous diaphragm such as, for example, one consisting of sintered glass wool.
Such a structure deviates considerably from the stretched shape of fluorescent tubes and therefore also requires another construction technique. The principal advantage of an increased light yield can therefore only be economically realized when the production expense does not greatly exceed the conventional expense of fluorescent tubes for AC current operation.
It is therefore an object of the present invention to produce a gas discharge tube having a structure which is suitable for DC current operation and which can be mass produced. The constructions presently suggested in the above-named letters patent and also in the article by A. Walz "Tests Relating to the Question of Possible Efficiency Degree Improvements in Electric Light Sources", (Magazine for Light Technique, in print) do this requirement justice only to a limited degree as they either require the building in of a complicated shaped electrode arrangement by a glass blower or they require the hermetically sealed connection of large-area glass parts and tube parts via a glass soldering technique which can only be produced under extreme difficulties when the glass solder is applied and the parts are positioned during the solder process, as experience has shown.
A discharge lamp suitable for DC operation essentially consists of a formed glass tube with an almost closed shape, said tube comprising the discharge space filled with the operating gas and/or the metal vapor and also the electrodes with their electrical inputs, two in the case of heated electrodes or one in the case of cold electrodes. They are conveyed in gas-tight manner through the glassy wall of the discharge space. Furthermore, one or more openings in the form of bored connecting pieces are expediently mounted in the vicinity of the electrodes. These bored connecting pieces are used for evacuating and filling the tubes with the operating gas and are melted off after the filling.
The nearly closed form of the gas discharge space for DC operation is required because the ions produced in the gas discharge wander towards the cathode in the direction of the electrical field cataphoresis and thereby effect a dropping of the light yield in long-time operation. If this effect is to be prevented without additional electrical or electronic expense, the anode and cathode spaces must have a connection which facilitates an exchange of the concentration differences formed by means of the cataphoresis. The connection has to be discharge-tight. This is obtained in the known manner by means of a small ratio between cross section and length in a capillary, or by means of a fiber felt. Accordingly the compound is not adulterated, for example, in the case of mercury vapor tubes, since the compound has to be held at a sufficiently high temperature during the operation of the lamp. This is most readily obtained by means of the spatial proximity of the connection to the electrodes. The nearly closed form of the electrode spaces therefore results.